The problem was that if something stopped working I couldn't be sure whether it was because I'd got something wrong, or had just jogged a connection.
Happily, prototype boards are so cheap that you can order something without worrying about the cost. This isn't even really a prototype yet, not all of the components are on the board. I just wanted the Pico, two SIDs (in this case swinsids) and the MIDI in circuit soldered firmly in place so that I could experiment with knobs, buttons and displays, and develop the software without fear of breaking it by jogging the desk!
To get you up-to-date if you haven't seen my ramblings about this, I'd love a box with MIDI input, SID chips inside, audio out, and simple controls. I'd like to be able to play a .mid file into it, or plug in a keyboard, with options for polyphonic or monophonic output. (I've done some of this using a C64 and MIDI cartridge, but it'll be good to have a standalone box.) I'd like to try a larger version with enough SIDs to be able to map each midi channel to a voice. Maybe in time I could add the components needed to support real SIDs, but being able to use ARMSIDs would be good as they're a great replacement. (I'm not sure that it's ethical to encourage anyone to use precious real SIDs in something like this.
Quick demo of the thing working (just one channel in, one voice playing). I've got Logic Pro playing a simple tune, with a USB-MIDI converter allowing me to plug that into this board.
If you're interested in the project, I'm going to use the hashtag #MIDISID on Twitter.